The present invention relates to a double face polishing apparatus capable of polishing both faces of a workpiece, e.g., silicon wafer.
A conventional double face polishing apparatus is disclosed in Japanese Patent Kokai Gazette No. 11-262862. In the apparatus, workpieces, e.g., silicon wafers, are clamped between an upper polishing plate and a lower polishing plate, and the polishing plates are rotated with supplying slurry to a polishing face of the lower polishing plate, so that both faces of the workpieces can be mechanically and chemically polished.
The conventional apparatus is shown in FIG. 6. The apparatus has a lower polishing plate 10, whose upper face acts as a polishing face, and an upper polishing plate 14, whose lower face acts as a polishing face and which is provided above the lower polishing plate 10 and can be vertically moved by a cylinder unit 12. The polishing plates 10 and 14 are rotated, about their own axes, in the opposite directions by a driving unit.
A carrier (not shown) having through-holes, in each of which the workpiece is held, is provided between the polishing plates 10 and 14. The carrier is rotated and orbited by a sun gear (not shown) and an internal gear (not shown). A ring-shaped duct 18, to which slurry is supplied from a slurry supply source 16, is provided to the upper polishing plate 14. A plurality of supply pipes 20 communicate slurry supply holes of the ring-shaped duct 18 to slurry supply holes 22 of the upper polishing plate 14 so as to supply the slurry to the lower polishing plate 10 via the supply pipes 20.
By supplying the slurry to the polishing face of the lower polishing plate 10, the workpieces can be mechanically and chemically polished. Preferably, the slurry is uniformly supplied to the lower polishing plate 10. Conventionally, the slurry supply holes 22 of the upper polishing plate 14 are radially arranged.
The slurry supplied onto the lower polishing plate 10 from the slurry supply holes 22, which are located an inner part of the upper polishing plate 14, is moved toward an outer part of the lower polishing plate 10 by a centrifugal force caused by rotation of the lower polishing plate 10. The slurry supply holes 22 are sparsely provided in the outer part of the lower polishing plate 10, but the slurry is moved from the inner part to the outer part, so that the slurry can be relatively uniformly supplied to the lower polishing plate 10.
However, in the above described conventional apparatus, an amount of the slurry to be supplied cannot be controlled. As shown in FIG. 6, lengths of the supply pipes 20 are not fixed. In the long pipes 20, flow resistance is great, and mid parts of the pipes are bent downward by their weights. Therefore, the slurry is apt to stay therein and cannot be smoothly supplied. On the other hand, in the short pipes 20, flow resistance is small and no slurry stays therein, so that the slurry can be smoothly supplied. Since the lengths of the supply pipes 20 are different, it is difficult to control the amount of the slurry.
In another conventional polishing apparatus, electromagnetic valves are respectively provided to the supply pipes 20 so as to precisely control the amount of slurry. However, it is troublesome to control each of the electromagnetic valves. Further, production cost must be increased.